1. Field
This disclosure relates generally to LDMOS transistors, and more specifically, to LDMOS transistors with stress in the channel.
2. Related Art
In laterally diffused metal oxide semiconductor (LDMOS) transistors, there is a tradeoff between resistivity, commonly known as Rdson, and high voltage breakdown. Breakdown can be increased by reducing the doping level of the high voltage N well that is for the drift region. This, however, causes the Rdson to increase. An approach for reducing Rdson that has been proposed is to introduce stress into the channel by forming a silicon layer over a thick epitaxial layer of silicon germanium (SiGe). One difficulty with this approach is that while depositing the underlying SiGe layer sufficiently thick for the SiGe layer to be relaxed, a high density of threading dislocations occurs. Some of these threading dislocations continue to grow into subsequently grown silicon causing defects in the silicon that lead to leakage and other potential problems. Further, other device types may be on the same integrated circuit as the LDMOS device and the grown SiGe layer may not be desirable for those other circuit types such as MOS transistors designed for logic operations and MOS transistors designed for analog operations. Also a thick SiGe layer is relatively expensive because of the time required to perform the growth.
Thus, there is a need for improving the tradeoff between Rdson and breakdown voltage in an LDMOS transistor while avoiding or reducing the impact of one or more of the problems described above.